The start_new_chapter callback should always execute when a chapter is started

[mojavelinux]

The start_new_chapter callback should always be executed when a chapter is started. Currently, this callback is not executed if the cursor is at the top of the page when the chapter begins. While this may be okay for the built-in behavior, it's a severe limitation for extensions that must perform other work at the chapter transitions.

The start_new_chapter callback will now be responsible for checking if the cursor is already at the top of a new page, including the built-in implementation.

[mojavelinux]

Here's a sample document that reproduces the issue:

= Document Title
:doctype: book

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----

== Chapter Title

content

The start_new_chapter method is never called due to the cursor placement after the listing block is rendered.